Error with Firefly/Grasshopper

I’m extremely new to Firefly and Grasshopper so my apologies if this is obvious. I created the following basic program (file attached) to read a value from a temperature sensor on an Arduino UNO board. However, I’m receiving the following error: “1. Solution exception:Object reference not set to an instance of an object.”

Could anyone please advise what I’m doing wrong? (11.0 KB)